Apparatus and Method for Injection Molding at Low Constant Pressure

ABSTRACT

A low constant pressure injection molding machine forms molded parts by injecting molten thermoplastic material into a mold cavity at low substantially constant pressures of 6,000 psi and less. As a result, the low constant pressure injection molding machine includes a mold formed of easily machineable material that is less costly and faster to manufacture than typical injection molds.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/488,564, filed May 20, 2011.

TECHNICAL FIELD

The present invention relates to apparatuses and methods for injectionmolding and, more particularly, to apparatuses and methods for producinginjection molded parts at low constant pressure.

BACKGROUND

Injection molding is a technology commonly used for high-volumemanufacturing of parts made of meltable material, most commonly of partsmade of thermoplastic polymers. During a repetitive injection moldingprocess, a plastic resin, most often in the form of small beads orpellets, is introduced to an injection molding machine that melts theresin beads under heat, pressure, and shear. The now molten resin isforcefully injected into a mold cavity having a particular cavity shape.The injected plastic is held under pressure in the mold cavity, cooled,and then removed as a solidified part having a shape that essentiallyduplicates the cavity shape of the mold. The mold itself may have asingle cavity or multiple cavities. Each cavity may be connected to aflow channel by a gate, which directs the flow of the molten resin intothe cavity. A molded part may have one or more gates. It is common forlarge parts to have two, three, or more gates to reduce the flowdistance the polymer must travel to fill the molded part. The one ormultiple gates per cavity may be located anywhere on the part geometry,and possess any cross-section shape such as being essentially circularor be shaped with an aspect ratio of 1.1 or greater. Thus, a typicalinjection molding procedure comprises four basic operations: (1) heatingthe plastic in the injection molding machine to allow it to flow underpressure; (2) injecting the melted plastic into a mold cavity orcavities defined between two mold halves that have been closed; (3)allowing the plastic to cool and harden in the cavity or cavities whileunder pressure; and (4) opening the mold halves to cause the part to beejected from the mold.

The molten plastic resin is injected into the mold cavity and theplastic resin is forcibly pushed through the cavity by an injectionelement of the injection molding machine until the plastic resin reachesthe location in the cavity furthest from the gate. The resulting lengthand wall thickness of the part is a result of the shape of the moldcavity.

While it may be desirous to reduce the wall thickness of injected moldedparts to reduce the plastic content, and thus cost, of the final part;reducing wall thickness using a conventional injection molding processcan be an expensive and a non-trivial task, particularly when designingfor wall thicknesses less than 15, 10, 5, 3, or 1.0 millimeter. As aliquid plastic resin is introduced into an injection mold in aconventional injection molding process, the material adjacent to thewalls of the cavity immediately begins to “freeze,” or solidify andcure. As the material flows through the mold, a boundary layer ofmaterial is formed against the sides of the mold. As the mold continuesto fill, the boundary layer continues to thicken, eventually closing offthe path of material flow and preventing additional material fromflowing into the mold. The plastic resin freezing on the walls of themold is exacerbated when the molds are cooled, a technique used toreduce the cycle time of each part and increase machine throughput.

There may also be a desire to design a part and the corresponding moldsuch that the liquid plastic resin flows from areas having the thickestwall thickness towards areas having the thinnest wall thickness.Increasing thickness in certain regions of the mold can ensure thatsufficient material flows into areas where strength and thickness isneeded. This “thick-to-thin” flow path requirement can make forinefficient use of plastic and result in higher part cost for injectionmolded part manufacturers because additional material must be moldedinto parts at locations where the material is unnecessary.

One method to decrease the wall thickness of a part is to increase thepressure of the liquid plastic resin as it is introduced into the mold.By increasing the pressure, the molding machine can continue to forceliquid material into the mold before the flow path has closed off.Increasing the pressure, however, has both cost and performancedownsides. As the pressure required to mold the component increases, themolding equipment must be strong enough to withstand the additionalpressure, which generally equates to being more expensive. Amanufacturer may have to purchase new equipment to accommodate theseincreased pressures. Thus, a decrease in the wall thickness of a givenpart can result in significant capital expenses to accomplish themanufacturing via conventional injection molding techniques.

Additionally, when the liquid plastic material flows into the injectionmold and rapidly freezes, the polymer chains retain the high levels ofstress that were present when the polymer was in liquid form. The frozenpolymer molecules retain higher levels of flow induced orientation whenmolecular orientation is locked in the part, resulting in a frozen-instressed state. These “molded-in” stresses can lead to parts that warpor sink following molding, have reduced mechanical properties, and havereduced resistance to chemical exposure. The reduced mechanicalproperties are particularly important to control and/or minimize forinjection molded parts such as thinwall tubs, living hinge parts, andclosures.

In an effort to avoid some of the drawbacks mentioned above, manyconventional injection molding operations use shear-thinning plasticmaterial to improve flow of the plastic material into the mold cavity.As the shear-thinning plastic material is injected into the mold cavity,shear forces generated between the plastic material and the mold cavitywalls tend to reduce viscosity of the plastic material, thereby allowingthe plastic material to flow more freely and easily into the moldcavity. As a result, it is possible to fill thinwall parts fast enoughto avoid the material freezing off before the mold is completely filled.

Reduction in viscosity is directly related to the magnitude of shearforces generated between the plastic material and the feed system, andbetween the plastic material and the mold cavity wall. Thus,manufacturers of these shear-thinning materials and operators ofinjection molding systems have been driving injection molding pressureshigher in an effort to increase shear, thus reducing viscosity.Typically, injection molding systems inject the plastic material in tothe mold cavity at melt pressures of 15,000 psi or more. Manufacturersof shear-thinning plastic material teach injection molding operators toinject the plastic material into the mold cavities above a minimum meltpressure. For example, polypropylene resin is typically processed atpressures greater than 6,000 psi (the recommended range from thepolypropylene resin manufacturers, is typically from greater than 6,000psi to about 15,000 psi. Resin manufacturers recommend not to exceed thetop end of the range. Press manufacturers and processing engineerstypically recommend processing shear thinning polymers at the top end ofthe range, or significantly higher, to achieve maximum potential shearthinning, which is typically greater than 15,000 psi, to extract maximumthinning and better flow properties from the plastic material. Shearthinning thermoplastic polymers generally are processed in the range ofover 6,000 psi to about 30,000 psi.

The molds used in injection molding machines must be capable ofwithstanding these high melt pressures. Moreover, the material formingthe mold must have a fatigue limit that can withstand the maximum cyclicstress for the total number of cycles a mold is expected to run over thecourse of its lifetime. As a result, mold manufacturers typically formthe mold from materials having high hardness, typically greater than 30Rc, and more typically greater than 50 Rc. These high hardness materialsare durable and equipped to withstand the high clamping pressuresrequired to keep mold components pressed against one another during theplastic injection process. These high hardness materials are also betterable to resist wear from the repeated contact between molding surfacesand polymer flow.

High production injection molding machines (i.e., class 101 and class102 molding machines) that produce thinwalled consumer productsexclusively use molds having a majority of the mold made from the highhardness materials. High production injection molding machines typicallyproduce 500,000 cycles per year or more. Industrial quality productionmolds must be designed to withstand at least 500,000 cycles per year,preferably more than 1,000,000 cycles per year, more preferably morethan 5,000,000 cycles per year, and even more preferably more than10,000,000 cycles per year. These machines have multi cavity molds andcomplex cooling systems to increase production rates. The high hardnessmaterials are more capable of withstanding the repeated high pressureclamping operations than lower hardness materials. However, highhardness materials, such as most tool steels, have relatively lowthermal conductivities, generally less than 20 BTU/HR FT ° F., whichleads to long cooling times as heat is transferred through from themolten plastic material through the high hardness material.

In an effort to reduce cycle times, typical high production injectionmolding machines having molds made of high hardness materials includerelatively complex internal cooling systems that circulate cooling fluidwithin the mold. These cooling systems accelerate cooling of the moldedparts, thus allowing the machine to complete more cycles in a givenamount of time, which increases production rates and thus the totalamount of molded parts produced. In some class 101, more than 1 or 2million cycles per year may be run, these molds are sometimes referredto as “ultra high productivity molds” Class 101 molds that run in 400ton or larger presses are sometimes referred to as “400 class” moldswithin the industry.

Another drawback to using high hardness materials for the molds is thathigh hardness materials, such as tool steels, generally are fairlydifficult to machine. As a result, known high throughput injection moldsrequire extensive machining time and expensive machining equipment toform, and expensive and time consuming post-machining steps to relievestresses and optimize material hardness.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 illustrates a schematic view of an injection molding machineconstructed according to the disclosure;

FIG. 2 illustrates one embodiment of a thin-walled part formed in theinjection molding machine of FIG. 1;

FIG. 3 is a cavity pressure vs. time graph for the injection moldingmachine of FIG. 1;

FIG. 4 is a cross-sectional view of one embodiment of a mold of theinjection molding machine of FIG. 1;

FIG. 5 is a perspective view of a feed system;

FIGS. 6A and 6B are top and front views of a naturally balanced feedsystem;

FIGS. 7A and 7B are top and front views of another naturally balancedfeed system;

FIG. 8 is a top view of an artificially balanced feed system that may beused in the injection molding machine of FIG. 1; and

FIGS. 9A and 9B are top views of non-balanced feed systems that may beused in the injection molding machine of FIG. 1.

DETAILED DESCRIPTION

Embodiments of the present invention generally relate to systems,machines, products, and methods of producing products by injectionmolding and more specifically to systems, products, and methods ofproducing products by low constant pressure injection molding.

The term “low pressure” as used herein with respect to melt pressure ofa thermoplastic material, means melt pressures in a vicinity of a nozzleof an injection molding machine of 6000 psi and lower.

The term “substantially constant pressure” as used herein with respectto a melt pressure of a thermoplastic material, means that deviationsfrom a baseline melt pressure do not produce meaningful changes inphysical properties of the thermoplastic material. For example,“substantially constant pressure” includes, but is not limited to,pressure variations for which viscosity of the melted thermoplasticmaterial do not meaningfully change. The term “substantially constant”in this respect includes deviations of approximately 30% from a baselinemelt pressure. For example, the term “a substantially constant pressureof approximately 4600 psi” includes pressure fluctuations within therange of about 6000 psi (30% above 4600 psi) to about 3200 psi (30%below 4600 psi). A melt pressure is considered substantially constant aslong as the melt pressure fluctuates no more than 30% from the recitedpressure.

Melt holder, as used herein, refers to the portion of an injectionmolding machine that contains molten plastic in fluid communication withthe machine nozzle. The melt holder is heated, such that a polymer maybe prepared and held at a desired temperature. The melt holder isconnected to a power source, for example a hydraulic cylinder orelectric servo motor, that is in communication with a central controlunit, and can be controlled to advance a diaphragm to force moltenplastic through the machine nozzle. The molten material then flowsthrough the runner system in to the mold cavity. The melt holder may bycylindrical in cross section, or have alternative cross sections thatwill permit a diaphragm to force polymer under pressures that can rangefrom as low as 100 psi to pressures 40,000 psi or higher through themachine nozzle. The diaphragm may optionally be integrally connected toa reciprocating screw with flights designed to plasticize polymermaterial prior to injection.

Referring to the figures in detail, FIG. 1 illustrates an exemplary lowconstant pressure injection molding apparatus 10 for producingthin-walled parts in high volumes (e.g., a class 101 or 102 injectionmold, or an “ultra high productivity mold”). The injection moldingapparatus 10 generally includes an injection system 12 and a clampingsystem 14. A thermoplastic material may be introduced to the injectionsystem 12 in the form of thermoplastic pellets 16. The thermoplasticpellets 16 may be placed into a hopper 18, which feeds the thermoplasticpellets 16 into a heated barrel 20 of the injection system 12. Thethermoplastic pellets 16, after being fed into the heated barrel 20, maybe driven to the end of the heated barrel 20 by a reciprocating screw22. The heating of the heated barrel 20 and the compression of thethermoplastic pellets 16 by the reciprocating screw 22 causes thethermoplastic pellets 16 to melt, forming a molten thermoplasticmaterial 24. The molten thermoplastic material is typically processed ata temperature of about 130° C. to about 410° C.

The reciprocating screw 22 forces the molten thermoplastic material 24,toward a nozzle 26 to form a shot comprising thermoplastic material,which will be injected into a mold cavity 32 of a mold 28. The moltenthermoplastic material 24 may be injected through a gate 30, whichdirects the flow of the molten thermoplastic material 24 to the moldcavity 32. The mold cavity 32 is formed between first and second moldparts 25, 27 of the mold 28 and the first and second mold parts 25, 27are held together under pressure by a press or clamping unit 34. Thepress or clamping unit 34 applies a clamping force in the range ofapproximately 1000 psi to approximately 6000 psi during the moldingprocess to hold the first and second mold parts 25, 27 together whilethe molten thermoplastic material 24 is injected into the mold cavity32. To support these clamping forces, the clamping system 14 may includea mold frame and a mold base, the mold frame and the mold base beingformed from a material having a surface hardness of more than about 165BHN and preferably less than 260 BHN, although materials having surfacehardness BHN values of greater than 260 may be used as long as thematerial is easily machineable, as discussed further below.

The mold may comprise a single mold cavity or a plurality of moldcavities. The plurality of mold cavities may comprise similar cavitiesor dissimilar cavities which will yield dissimilar parts. The mold mayalso comprises grouped family of dissimilar cavities.

Once the shot comprising molten thermoplastic material 24 is injectedinto the mold cavity 32, the reciprocating screw 22 stops travelingforward. The molten thermoplastic material 24 takes the form of the moldcavity 32 and the molten thermoplastic material 24 cools inside the mold28 until the thermoplastic material 24 solidifies. Once thethermoplastic material 24 has solidified, the press 34 releases thefirst and second mold parts 25, 27, the first and second mold parts 25,27 are separated from one another, and the finished part may be ejectedfrom the mold 28. The mold 28 may include a plurality of mold cavities32 to increase overall production rates.

A controller 50 is communicatively connected with a sensor 52 and ascrew control 36. The controller 50 may include a microprocessor, amemory, and one or more communication links. The controller 50 may beconnected to the sensor 52 and the screw control 36 via wiredconnections 54, 56, respectively. In other embodiments, the controller50 may be connected to the sensor 52 and screw control 56 via a wirelessconnection, a mechanical connection, a hydraulic connection, a pneumaticconnection, or any other type of communication connection known to thosehaving ordinary skill in the art that will allow the controller 50 tocommunicate with both the sensor 52 and the screw control 36. There maybe intermediary operative units in the communications path between thesensor, the controller, and the screw control.

In the embodiment of FIG. 1, the sensor 52 is a pressure sensor thatmeasures (directly or indirectly) melt pressure of the moltenthermoplastic material 24 in the nozzle 26. The sensor 52 generates anelectrical signal that is transmitted to the controller 50. Thecontroller 50 then commands the screw control 36 to advance the screw 22at a rate that maintains a substantially constant melt pressure of themolten thermoplastic material 24 in the nozzle 26. While the sensor 52may directly measure the melt pressure, the sensor 52 may measure othercharacteristics of the molten thermoplastic material 24, such astemperature, viscosity, flow rate, etc, that are indicative of meltpressure. Likewise, the sensor 52 need not be located directly in thenozzle 26, but rather the sensor 52 may be located at any locationwithin the injection system 12 or mold 28 that is fluidly connected withthe nozzle 26. The sensor 52 need not be in direct contact with theinjected fluid and may alternatively be in dynamic communication withthe fluid and able to sense the pressure of the fluid and/or other fluidcharacteristics. If the sensor 52 is not located within the nozzle 26,appropriate correction factors may be applied to the measuredcharacteristic to calculate the melt pressure in the nozzle 26. In yetother embodiments, the sensor 52 need not be disposed at a locationwhich is fluidly connected with the nozzle. Rather, the sensor couldmeasure clamping force generated by the clamping system 14 at a moldparting line between the first and second mold parts 25, 27. In oneaspect the controller 50 may maintain the pressure according to theinput from sensor 52.

A sensor may be located near the end of fill in the mold cavity. Thissensor may provide an indication of when the mold front is approachingthe end of fill in the cavity. The sensor may sense pressure,temperature, optically, or other means of identifying the presence ofthe polymer. When pressure is measured by the sensor, this measure canbe used to communicate with the central control unit to provide a target“packing pressure” for the molded component. The signal generated by thesensor can be used to control the molding process, such that variationsin material viscosity, mold temperatures, melt temperatures, and othervariations influencing filling rate, can be adjusted for by the centralcontrol unit. These adjustments can be made immediately during themolding cycle, or corrections can be made in subsequent cycles.Furthermore, several readings can be averaged over a number of cyclesthen used to make adjustments to the molding process by the centralcontrol unit. In this way, the current injection cycle can be correctedbased on measurements occurring during one or more cycles at an earlierpoint in time. In one embodiment, sensor readings can be averaged overmany cycles so as to achieve process consistency.

Although an active, closed loop controller 50 is illustrated in FIG. 1,other pressure regulating devices may be used instead of the closed loopcontroller 50. For example, a pressure regulating valve (not shown) or apressure relief valve (not shown) may replace the controller 50 toregulate the melt pressure of the molten thermoplastic material 24. Morespecifically, the pressure regulating valve and pressure relief valvecan prevent overpressurization of the mold 28. Another alternativemechanism for preventing overpressurization of the mold 28 is an alarmthat is activated when an overpressurization condition is detected.

Turning now to FIG. 2, an example molded part 100 is illustrated. Themolded part 100 is a thin-walled part. Molded parts are generallyconsidered to be thin-walled when a length of a flow channel L dividedby a thickness of the flow channel T is greater than 100 (i.e.,L/T>100). In some injection molding industries, thin-walled parts may bedefined as parts having an L/T>200, or an L/T>250. The length of theflow channel L is measured from a gate 102 to a flow channel end 104.Thin-walled parts are especially prevalent in the consumer productsindustry.

Thin-walled parts present certain obstacles in injection molding. Moldedparts are generally considered to be thin-walled when a length of a flowchannel L divided by a thickness of the flow channel T is greater than100 (i.e., L/T>100). For mold cavities having a more complicatedgeometry, the L/T ratio may be calculated by integrating the T dimensionover the length of the mold cavity 32 from a gate 102 to the end of themold cavity 32, and determining the longest length of flow from the gate102 to the end of the mold cavity 32. The L/T ratio can then bedetermined by dividing the longest length of flow by the average partthickness.

For example, the thinness of the flow channel tends to cool the moltenthermoplastic material before the material reaches the flow channel end104. When this happens, the thermoplastic material freezes off and nolonger flows, which results in an incomplete part. To overcome thisproblem, traditional injection molding machines inject the moltenthermoplastic material at very high pressures, typically greater than15,000 psi, so that the molten thermoplastic material rapidly fills themold cavity before having a chance to cool and freeze off. This is onereason that manufacturers of the thermoplastic materials teach injectingat very high pressures. Another reason traditional injection moldingmachines inject at high pressures is the increased shear, whichincreases flow characteristics, as discussed above. These very highinjection pressures require the use of very hard materials to form themold 28 and the feed system.

Traditional injection molding machines use tool steels or other hardmaterials to make the mold. While these tool steels are robust enough towithstand the very high injection pressures, tool steels are relativelypoor thermal conductors. As a result, very complex cooling systems aremachined into the molds to enhance cooling times when the mold cavity isfilled, which reduces cycle times and increases productivity of themold. However, these very complex cooling systems add great time andexpense to the mold making process.

The inventors have discovered that shear-thinning thermoplastics (evenminimally shear-thinning thermoplastics) may be injected into the mold28 at low, substantially constant, pressure without any significantadverse affects. Examples of these materials include but are not limitedto polymers and copolymers comprised of, polypropylene, polyethylene,thermoplastic elastomers, polyester, polystyrene, polycarbonate,poly(acrylonitrile-butadiene-styrene), poly(latic acid),polyhydroxyalkanoate, polyamides, polyacetals, ethylene-alpha olefinrubbers, and styrene-butadiene-stryene block copolymers. In fact, partsmolded at low, substantially constant, pressures exhibit some superiorproperties as compared to the same part molded at a conventional highpressure. This discovery directly contradicts conventional wisdom withinthe industry that teaches higher injection pressures are better. Withoutbeing bound by theory, it is believed that injecting the moltenthermoplastic material into the mold 28 at low, substantially constant,pressures creates a continuous flow front of thermoplastic material thatadvances through the mold from a gate to a farthest part of the moldcavity. By maintaining a low level of shear, the thermoplastic materialremains liquid and flowable at much lower temperatures and pressuresthan is otherwise believed to be possible in conventional high pressureinjection molding systems.

Exemplary thermoplastic resins together with their recommended operatingpressure ranges are provided in the following chart:

Injection Pressure Material Brand Material Full Name Range (PSI) CompanyName PP Polypropylene 10000-15000 RTP RTP 100 series ImagineeringPolypropylene Plastics Nylon 10000-18000 RTP RTP 200 series ImagineeringNylon Plastics ABS Acrylonitrile  8000-20000 Marplex Astalac ABSButadiene Styrene PET Polyester  5800-14500 Asia AIE PET 401FInternational Acetal Co-  7000-17000 API Kolon Kocetal polymer PCPolycarbonate 10000-15000 RTP RTP 300 series Imagineering PolycarbonatePlastics PS Polystyrene 10000-15000 RTP RTP 400 series ImagineeringPlastics SAN Styrene 10000-15000 RTP RTP 500 series AcrylonitrileImagineering Plastics PE LDPE & 10000-15000 RTP RTP 700 Series HDPEImagineering Plastics TPE Thermoplastic 10000-15000 RTP RTP 1500Elastomer Imagineering series Plastics PVDF Polyvinylidene 10000-15000RTP RTP 3300 Fluoride Imagineering series Plastics PTI Poly- 10000-15000RTP RTP 4700 trimethylene Imagineering series Terephthalate Plastics PBTPolybutylene 10000-15000 RTP RTP 1000 Terephthalate Imagineering seriesPlastics PLA Polylactic Acid  8000-15000 RTP RTP 2099 Imagineeringseries Plastics

Turning now to FIG. 3, a typical pressure-time curve for a conventionalhigh pressure injection molding process is illustrated by the dashedline 200. By contrast, a pressure-time curve for the disclosed lowconstant pressure injection molding machine is illustrated by the solidline 210.

In the conventional case, melt pressure is rapidly increased to wellover 15,000 psi and then held at a relatively high pressure, more than15,000 psi, for a first period of time 220. The first period of time 220is the fill time in which molten plastic material flows into the moldcavity. Thereafter, the melt pressure is decreased and held at a lower,but still relatively high pressure, 10,000 psi or more, for a secondperiod of time 230. The second period of time 230 is a packing time inwhich the melt pressure is maintained to ensure that all gaps in themold cavity are back filled. The mold cavity in a conventional highpressure injection molding system is filled from the end of the flowchannel back to towards the gate. As a result, plastic in various stagesof solidification are packed upon one another, which may causeinconsistencies in the finished product, as discussed above. Moreover,the conventional packing of plastic in various stages of solidificationresults in some non-ideal material properties, for example, molded-instresses, sink, and non-optimal optical properties.

The constant low pressure injection molding system, on the other hand,injects the molten plastic material into the mold cavity at asubstantially constant low pressure for a single time period 240. Theinjection pressure is less than 6,000 psi. By using a substantiallyconstant low pressure, the molten thermoplastic material maintains acontinuous melt front that advances through the flow channel from thegate towards the end of the flow channel. Thus, the plastic materialremains relatively uniform at any point along the flow channel, whichresults in a more uniform and consistent finished product. By fillingthe mold with a relatively uniform plastic material, the finished moldedparts form crystalline structures that have better mechanical andoptical properties than conventionally molded parts. Moreover, the skinlayers of parts molded at low constant pressures exhibit differentcharacteristics than skin layers of conventionally molded parts. As aresult, the skin layers of parts molded under low constant pressure canhave better optical properties than skin layers of conventionally moldedparts.

By maintaining a substantially constant and low (e.g., less than 6000psi) melt pressure within the nozzle, more machineable materials may beused to form the mold 28. For example, the mold 28 illustrated in FIG. 1may be formed of a material having a milling machining index of greaterthan 100%, a drilling machining index of greater than 100%, a wire EDMmachining index of greater than 100%, a graphite sinker EDM machiningindex of greater than 200%, or a copper sinker EDM machining index ofgreater than 150%. The machining indexes are based upon milling,drilling, wire EDM, and sinker EDM tests of various materials. The testmethods for determining the machining indices are explained in moredetail below. Examples of machining indexes for a sample of materials iscompiled below in Table 1.

TABLE 1 Machining Technology Milling Drilling Sinker EDM- Sinker EDM-Spindle Spindle Wire EDM Graphite Copper Load Index % Load Index % timeIndex % time Index % time Index % Material 1117* 0.72 100% 0.32 100%9:34 100% 0:14:48 100% 0:24:00 100% 6061 Al 0.50 144% 0.20 160% 4:46201% 0:05:58 248% 0:15:36 154% 7075 Al 0.55 131% 0.24 133% 4:48 199%0:05:20 278% 0:12:27 193% Alcoa QC-10 Al 0.56 129% 0.24 133% 4:47 200%0:05:11 286% 0:12:21 194% 4140 0.92 78% 0.37 86% 9:28 101% 0:09:36 154%0:19:20 124% 420 SS 1.36 53% 0.39 82% 8:30 113% 0:10:12 145% 0:23:20103% A2 0.97 74% 0.45 71% 8:52 108% 0:08:00 185% 0:20:12 119% S7 1.2060% 0.43 74% 9:03 106% 0:12:53 115% 0:20:58 114% P20 1.10 65% 0.38 84%9:26 101% 0:11:47 126% 0:20:30 117% PX5 1.12 64% 0.37 86% 9:22 102%0:12:37 117% 0:23:18 103% Moldmax HH 0.80 90% 0.36 89% 6:00 159% 6:59:35   4% 1 0:43:38   55% 3 Ampcoloy 944 0.62 116% 0.32 100% 6:53 139%3:13:41    8% 2 0:30:21   79% 4 *1117 is the benchmark material for thistest. Published data references 1212 carbon steel as the benchmarkmaterial. 1212 was not readily available. Of the published data, 1117was the closest in composition and machining index percentage (91%). 1Significant graphite electrode wear: ~20% 2 graphite electrode wear:~15% 3 Cu electrode wear: ~15% 4 Cu electrode wear: ~3%

Using easily machineable materials to form the mold 28 results ingreatly decreased manufacturing time and thus, a decrease inmanufacturing costs. Moreover, these machineable materials generallyhave better thermal conductivity than tool steels, which increasescooling efficiency and decreases the need for complex cooling systems.

When forming the mold 28 of these easily machineable materials, it isalso advantageous to select easily machineable materials having goodthermal conductivity properties. Materials having thermal conductivitiesof more than 30 BTU/HR FT ° F. are particularly advantageous. Forexample easily machineable materials having good thermal conductivitiesinclude, but are not limited to, Alcoa QC-10, Alcan Duramold 500, andHokotol (available from Aleris). Materials with good thermalconductivity more efficiently transmit heat from the thermoplasticmaterial out of the mold. As a result, more simple cooling systems maybe used. Additionally, non-naturally balanced feed systems are alsopossible for use in the constant low pressure injection molding machinesdescribed herein.

One example of a multi-cavity mold 28 is illustrated in FIG. 4.Multi-cavity molds generally include a feed manifold 60 that directsmolten thermoplastic material from the nozzle 26 to the individual moldcavities 32. The feed manifold 60 includes a sprue 62, which directs themolten thermoplastic material into one or more runners or feed channels64. Each runner may feed multiple mold cavities 32. In many highcapacity injection molding machines, the runners are heated to enhanceflowability of the molten thermoplastic material. Because viscosity ofthe molten thermoplastic material is very sensitive to shear andpressure variations at high pressures (e.g., above 10,000 psi),conventional feed manifolds are naturally balanced to maintain uniformviscosity. Naturally balanced feed manifolds are manifolds in whichmolten thermoplastic material travels an equal distance from the sprueto any mold cavity. Moreover, the cross-sectional shapes of each flowchannel are identical, the number and type of turns are identical, andthe temperatures of each flow channel are identical. Naturally balancedfeed manifolds allow the mold cavities to be filled simultaneously sothat each molded part has identical processing conditions and materialproperties. Naturally balanced feed manifolds are expensive tomanufacture and limit mold designs somewhat.

FIG. 5 illustrates an example of a naturally balanced feed manifold 60.The naturally balanced feed manifold 60 includes a first flow path 70from the sprue 62 to a first junction 72 where the first flow path 70splits into second and third flow paths 74, 76, the second flow pathterminating at a second gate 78 a and the third flow path 76 terminatingat a third gate 78 b each gate serving an individual mold cavity (notshown in FIG. 5). Molten thermoplastic material flowing from the sprue62 to either the second gate 78 a or the third gate 78 b travels thesame distance, experiences the same temperatures, and is subjected tothe same cross-sectional flow areas. As a result, each mold cavity isfilled simultaneously with molten thermoplastic material havingidentical physical properties.

FIGS. 6A and 6B illustrate the naturally balanced manifold 60schematically. The naturally balanced manifold 60 of FIGS. 6A and 6B isa multi-tier manifold. Each flow path 74, 76 has identicalcharacteristics at identical locations along the flow path. For example,after the junction 72, each flow path narrows at the same distance.Moreover, each flow path serves an identical number of mold cavities 32.Naturally balanced flow manifolds 60 are critical to high pressureinjection molding machines to maintain identical plastic flow propertiesand to ensure uniform parts.

FIGS. 7A and 7B illustrate another naturally balanced manifold 60. Thenaturally balanced manifold 60 of FIGS. 7A and 7B is a single tiermanifold.

By contrast, FIGS. 8, 9A, and 9B illustrate non-naturally balancedmanifolds with FIG. 8 illustrating an artificially balanced manifold andFIGS. 9A and 9B illustrating non-balanced manifolds.

The low constant pressure injection molding machine disclosed hereinallows artificially balanced manifolds, and even unbalanced manifolds,to be used because thermoplastic materials injected at low constantpressure are not as sensitive to pressure differences or sheardifferences due to flow channel characteristic differences. In otherwords, the thermoplastic materials injected at low constant pressureretain nearly identical material and flow properties regardless ofdifferences in flow channel length, cross-sectional area, ortemperature. As a result, mold cavities may be filed sequentiallyinstead of simultaneously.

The artificially balanced manifold 160 of FIG. 8 includes a sprue 62, afirst flow channel 174, and a second flow channel 176. The first flowchannel 174 terminates at a first gate 178 a and the second flow channel176 terminates at a second gate 178 b. The first flow channel 174 isshorter than the second flow channel 178 in this embodiment. Theartificially balanced manifold 160 varies some other parameter of theflow channel (e.g., cross-sectional area or temperature) so that thematerial flowing through the manifold 160 provides balanced flow to eachcavity, similar to a naturally balanced manifold. In other words,thermoplastic material flowing through the first flow channel 174 willhave about equal melt pressure to thermoplastic material flowing throughthe second flow channel 176. Because artificially balanced, orunbalanced, feed manifolds can include flow channels of differentlengths, an artificially balanced, or unbalanced, feed manifold can makemuch more efficient use of space. Moreover, the feed channels andcorresponding heater band channels can be machined more efficiently.Furthermore, naturally balanced feed manifolds are limited to moldshaving distinct, even numbers of mold cavities (e.g., 2, 4, 8, 16, 32,etc.). Artificially balanced, and unbalanced, feed manifolds may bedesigned to deliver molten thermoplastic material to any number of moldcavities.

The artificially balanced feed manifold 160 may also be constructed of amaterial having high thermal conductivity to enhance heat transfer tothe molten thermoplastic material in hot runners, thus enhancing flow ofthe thermoplastic material. More specifically, the artificially balancedfeed manifold 160 may be constructed of the same material as the mold tofurther reduce material costs and enhance heat transfer within theentire system.

FIGS. 9A and 9B illustrate non-balanced manifolds 260. The non-balancedmanifolds 260 may include an odd number of mold cavities 232, and/orflow channels having different cross-sectional shapes, different numberand type of turns, and/or the different temperatures. Moreover, thenon-balanced manifolds 260 may feed mold cavities having differentsizes, and or shapes, as illustrated in FIG. 9B.

Drilling and Milling Machineability Index Test Methods

The drilling and milling machineability indices listed above in Table 1were determined by testing the representative materials in carefullycontrolled test methods, which are described below.

The machineability index for each material was determined by measuringthe spindle load needed to drill or mill a piece of the material withall other machine conditions (e.g., stock feed rate, spindle rpm, etc.)being held constant between the various materials. Spindle load isreported as a ratio of the measured spindle load to the maximum spindletorque load of 75 ft-lb at 1400 rpm for the drilling or milling device.The index percentage was calculated as a ratio between the spindle loadfor 1117 steel to the spindle load for the test material.

The test milling or drilling machine was a Hass VF-3 Machining Center.

Drilling Conditions

TABLE 2 Spot Drill 120 degree 0.5″ diameter, drilled to 0.0693″ depthDrill Bit 15/32″ diameter high speed steel uncoated jobber length bitSpindle Speed 1200 rpm Depth of Drill 0.5″ Drill Rate 3 in/min Other Nochip break routine used

Milling Conditions

TABLE 3 Mill 0.5″ diameter 4 flute carbide flat bottom end mill,uncoated (SGS part # 36432 www.sgstool.com) Spindle Speed 1200 rpm Depthof Cut 0.5″ Stock Feed Rate 20 in/min

For all tests “flood blast” cooling was used. The coolant was Koolrite2290.

EDM Machineability Index Test Methods

The graphite and copper sinker EDM machineability indices listed abovein Table 1 were determined by testing the representative materials in acarefully controlled test method, which is described below.

The EDM machineability index for the various materials were determinedby measuring the time to burn an area (specifics below) into the varioustest metals. The machineability index percentage was calculated as theratio of the time to burn into 1117 steel to time required to burn thesame area into the other test materials.

Wire EDM

TABLE 4 Equipment Fanuc OB Wire 0.25 mm diameter hard brass Cut 1″ thick× 1″ length (1 sq. ″) Parameters Used Fanuc on board artificialintelligence, override at 100%

Sinker EDM—Graphite

TABLE 5 Equipment Ingersoll Gantry 800 with Mitsubishi EX ControllerWire System 3R pre-mounted 25 mm diameter Poco EDM 3 graphite Cut 0.1″ Zaxis plunge Parameters Used Mitsubishi CNC controls with FAP EX SeriesTechnology

Sinker EDM—Copper

TABLE 6 Equipment Ingersoll Gantry 800 with Mitsubishi EX ControllerWire System 3R pre-mounted 25 mm diameter Tellurium Copper Cut 0.1″ Zaxis plunge Parameters Used Mitsubishi CNC controls with FAP EX SeriesTechnology

The disclosed low constant pressure injection molding machinesadvantageously employ molds constructed from easily machineablematerials. As a result, the disclosed low constant pressure injectionmolding machines are less expensive and faster to produce. Additionally,the disclosed low constant pressure injection molding machines arecapable of employing more flexible support structures and more adaptabledelivery structures, such as wider platen widths, increased tie barspacing, elimination of tie bars, lighter weight construction tofacilitate faster movements, and non-naturally balanced feed systems.Thus, the disclosed low constant pressure injection molding machines maybe modified to fit delivery needs and are more easily customizable forparticular molded parts.

It is noted that the terms “substantially,” “about,” and“approximately,” unless otherwise specified, may be utilized herein torepresent the inherent degree of uncertainty that may be attributed toany quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. Unless otherwise defined herein, the terms“substantially,” “about,” and “approximately” mean the quantitativecomparison, value, measurement, or other representation may fall within20% of the stated reference.

It should now be apparent that the various embodiments of the productsillustrated and described herein may be produced by a low constantpressure injection molding process. While particular reference has beenmade herein to products for containing consumer goods or consumer goodsproducts themselves, it should be apparent that the low constantpressure injection molding method discussed herein may be suitable foruse in conjunction with products for use in the consumer goods industry,the food service industry, the transportation industry, the medicalindustry, the toy industry, and the like. Moreover, one skilled in theart will recognize the teachings disclosed herein may be used in theconstruction of stack molds, multiple material molds includingrotational and core back molds, in combination with in-mold decoration,insert molding, in mold assembly, and the like. Moreover, one skilled inthe art will recognize the teachings disclosed herein may be used in theconstruction of stack molds, multiple material molds includingrotational and core back molds, in combination with in-mold decoration,insert molding, in mold assembly, and the like.

All documents cited in the Detailed Description of the Invention are, inrelevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention. To the extent that any meaning ordefinition of a term in this written document conflicts with any meaningor definition of the term in a document incorporated by reference, themeaning or definition assigned to the term in this written documentshall govern.

While particular embodiments have been illustrated and described herein,it should be understood that various other changes and modifications maybe made without departing from the spirit and scope of the claimedsubject matter. Moreover, although various aspects of the claimedsubject matter have been described herein, such aspects need not beutilized in combination. It is therefore intended that the appendedclaims cover all such changes and modifications that are within thescope of the claimed subject matter.

1. An injection molding apparatus comprising: a melt holder forpressurizing molten plastic prior to injection into a mold having aplurality of mold cavities; a sensor in dynamic communication with themelt holder for sensing a characteristic of the molten plastic; and acontroller in communication with the sensor, the controller receiving asignal from the sensor, the signal being indicative of a melt pressureof the molten plastic entering at least one mold cavity in the pluralityof mold cavities, the controller further being in communication with aninjection element, the injection element applying a force to the moltenplastic to advance the molten plastic from the melt holder into themold, wherein the controller controls the injection element to maintaina substantially constant melt pressure entering the at least one moldcavity of less than 6000 psi, and the mold has an average thermalconductivity of more than 30 BTU/HR FT ° F. and the mold has at leastone of a milling machining index of greater than 100%, a drillingmachining index of greater than 100%, and a wire EDM machining index ofgreater than 100%.
 2. The injection molding apparatus of claim 1,wherein the at least one mold cavity in the plurality of mold cavitiesis a thin-walled mold cavity having an L/T>100.
 3. The injection moldingapparatus of claim 1, further comprising an artificially balanced moltenplastic feed system.
 4. The injection molding apparatus of claim 1,wherein sensor generates an electrical signal.
 5. The injection moldingapparatus of claim 1, wherein the sensor generates a mechanical signal.6. The injection molding apparatus of claim 1, wherein the sensorgenerates a hydraulic signal.
 7. The injection molding apparatus ofclaim 1, wherein the sensor generates a pneumatic signal.
 8. Theinjection molding apparatus of claim 1, further comprising a mold frameand a mold base, wherein at least one of the mold frame and the moldbase is made from a material having a surface hardness of greater than165 BHN and less than 260 BHN.
 9. The injection molding apparatus ofclaim 1, further comprising a hot runner feed system constructed of amaterial having a thermal conductivity that is substantially equal tothe average thermal conductivity of the mold.
 10. The injection moldingapparatus of claim 9, wherein the hot runner feed system is directlyconnected to a gate that is fluidly connected with the at least one moldcavity.
 11. The injection molding apparatus of claim 1, wherein the moldhas a sinker EDM machining index of greater than 200%.
 12. The injectionmolding apparatus of claim 11, wherein the mold has a sinker EDMmachining index of between 200% and 1000%.
 13. The injection moldingapparatus of claim 1, wherein the mold comprises at least four moldcavities.
 14. An injection molding apparatus comprising: a mold havingat least one mold cavity, the mold having an average thermalconductivity more than 30 BTU/HR FT ° F., and a sinker EDM machiningindex of greater than 200%; a melt holder for pressurizing moltenplastic prior to injection into the mold; a sensor in communication withthe melt holder for sensing a characteristic of the molten plastic; anda controller in communication with the pressure sensor, the controllerreceiving a signal from the sensor, the signal being indicative of amelt pressure of the molten plastic entering the mold cavity, thecontroller further being in communication with an injection element, theinjection element applying a force to the molten plastic to advance themolten plastic from the melt holder into the mold, wherein thecontroller controls the injection element to maintain a melt pressure ofthe molten plastic entering the mold cavity of less than 6000 psi. 15.The injection molding apparatus of claim 14 wherein the controllercontrols the injection element to maintain a substantially constant meltpressure.
 16. An injection molding apparatus comprising: a mold havingat least one mold cavity, the mold having an average thermalconductivity more than 30 BTU/HR FT ° F., and the mold having at leastone of a milling machining index of greater than 100%, a drillingmachining index of greater than 100%, and a wire EDM machining index ofgreater than 100%; a melt holder for pressurizing molten plastic priorto injection into the mold; a sensor in communication with the meltholder for sensing a characteristic of the molten plastic indicative ofa melt pressure of the molten plastic in a nozzle of the melt holder,just prior to entering the mold; and a controller in communication withthe sensor, the controller receiving a signal from the pressure sensor,the controller further being in communication with an injection element,the injection element applying a force to the molten plastic to advancethe molten plastic from the melt holder into the mold, wherein thecontroller controls the injection element to maintain a melt pressure ofless than 6000 psi.
 17. The injection molding apparatus of claim 16,wherein the controller controls the injection element to maintain asubstantially constant melt pressure.
 18. An injection molding apparatuscomprising: a mold having at least one mold cavity, the mold having anaverage thermal conductivity more than 30 BTU/HR FT ° F., and at leastone of a thin-walled mold cavity (e.g., L/T>100), at least four moldcavities, and an artificially balanced molten plastic feed system; amelt holder for pressurizing molten plastic prior to injection into themold; a sensor in communication with the melt holder for sensing acharacteristic of the molten plastic indicative of a melt pressure ofthe molten plastic in a nozzle of the melt holder just before enteringthe mold; and a controller in communication with the sensor, thecontroller receiving a signal from the sensor, the controller furtherbeing in communication with an injection element, the injection elementapplying a force to the molten plastic to advance the molten plasticfrom the melt holder into the mold, wherein the controller controls theinjection element to maintain a melt pressure of less than 6000 psi. 19.The injection molding apparatus of claim 18, wherein the controllercontrols the injection element to maintain a substantially constant meltpressure.
 20. An injection molding apparatus comprising: a mold havingat least one mold cavity, the mold being formed from a material having asurface hardness less than 30 Rc and the mold having an average thermalconductivity of more than 30 BTU/HR FT ° F.; a melt holder forpressurizing molten plastic prior to injecting the molten plastic intothe mold; a guided ejection system; and a controller that controls aninjection element in the melt holder so that the injection elementadvances molten plastic into the at least one mold cavity at a meltpressure of less than 6000 psi.